A wearable system for pre-impact fall detection

Abstract

Unique features of body segment kinematics in falls and activities of daily living (ADL) are applied to make automatic detection of a fall in its descending phase, prior to impact, possible. Fall-related injuries can thus be prevented or reduced by deploying fall impact reduction systems, such as an inflatable airbag for hip protection, before the impact. In this application, the authors propose the following hypothesis: “Thigh segments normally do not exceed a certain threshold angle to the side and forward directions in ADL, whereas this abnormal behavior occurs during a fall activity”. Torso and thigh wearable inertial sensors (3D accelerometer and 2D gyroscope) are used and the whole system is based on a body area network (BAN) for the comfort of the wearer during a long term application. The hypothesis was validated in an experiment with 21 young healthy volunteers performing both normal ADL and fall activities. Results show that falls could be detected with an average lead-time of 700 ms before the impact occurs, with no false alarms (100% specificity), a sensitivity of 95.2%. This is the longest lead-time achieved so far in pre-impact fall detection.

Introduction

Falls are a major care and cost burden to health and social services world-wide (Annekenny and O’Shea, 2002). Falls have traditionally been recognized as one of the “giants” for geriatric medicine, reflected from the high incidence of falls, common adverse sequelae such as fractures, and the major psychological impact. Among the causes of falls, fainting (syncope) is one common factor in older people and also related to unexplained and recurrent falls (McIntosh et al., 1993). Syncopal episodes or fainting related falls are unwitnessed in 40–60% of older people over 65 (McIntosh et al., 1993) and cause considerable mortality and morbidity among this age group.

Even though most falls produce no serious injury, only 1–2% of falls result in hip fractures (Hayes et al., 1996), 5–10% of community-dwelling older adults who fall each year do sustain a serious injury such as fracture, head injury, or serious laceration (Nevitt et al., 1991; Tinetti et al., 1995). Of all the fall-related traumas, fractures of the neck and trochanteric regions of the femur, the major bone in the hip joint, are currently one of the most serious health care problems faced in aging populations (Marks et al., 2003). Most hip fractures (60–99%) are related to direct trauma to the hip (Chapuy et al., 1992; Cummings and Nevitt, 1989; Hipp et al., 1991; Lauritzen and Askegaard, 1992). An investigation performed by Smeesters et al. showed that at any gait speed, that faint falls resulted in a greater number of sideways falls with impact near the hip (Smeesters et al., 2001). In these scenarios, the most promising prevention strategies for faint fall involves the identification of individuals who are at increased risk and the implementation of appropriate interventions, these include physical restraint (Gross et al., 1990), investigation of fall-related fractures prevention strategies (Smeesters et al., 2001; Van den Kroonenberg et al., 1996; Yamamoto et al., 2006), study of characteristics and risk factors of syncope (Kenny and O’Shea, 2002; Peczalski et al., 2006), and multi-factorial risk assessment and management (Sjösten et al., 2007; Weatherall, 2004).

In fall intervention strategies, one of the key concerns in preventing or reducing the severity of injury in the elderly is to detect the fall in its descending phase (Hayes et al., 1996) before the impact (pre-impact fall detection). A few groups have attempted to detect falls prior to impact (Bourke et al., 2008; Nyan et al., 2006; Wu, 2000). Some researchers have investigated inflatable hip protectors to cushion the fall prior to impact (Davidson, 2004; Lockhart, 2006; Ulert, 2002). Wu implemented pre-impact fall detection by thresholding the horizontal and vertical velocity profiles of the trunk using motion analysis system. Wu showed that falls can be distinguished from activities of daily living (ADL) with 300–400 ms lead-time before the impact (Wu, 2000). Nyan et al. used three gyroscope sensors at three different locations, the sternum, front of the waist and under the arm, for fall pre-impact detection. Nyan achieved 100% sensitivity with approximately 200 ms lead-time before the impact; however, 16% of ADL events tested were misinterpreted as falls (Nyan et al., 2006). In addition, at the instant when fall is detected, the angle of body configuration from the vertical axis is 40–54° (Nyan et al., 2006). Bourke et al. investigated pre-impact detection of falls by thresholding the vertical velocity of the trunk. An optical motion capture system and an inertial sensor unit consisting of a tri-axial accelerometer and a tri-axial gyroscope were used in their experiments. The inertial sensor was located on the chest of the body using a harness. Falls can be distinguished from normal ADL, with 100% accuracy and with an average of 323 ms prior to trunk impact and 140 ms prior to knee impact, in that subject group (Bourke et al., 2008).

In pre-impact fall detection, if a fall can be detected in its earliest stage, i.e., in the descent phase (Hayes et al., 1996), more efficient impact reduction systems can be implemented with a longer lead-time for injury minimization.

This paper presents the implementation and clinical trial results of a wearable pre-impact fall detection prototype, using inertial sensors to detect faint falls in its incipience. The approach is based on the characteristics of angular movements of the thigh and torso segments in falls and ADL. The authors hypothesize the following statement: “Thigh segments normally do not exceed a certain threshold angle to the side and forward directions in ADL, whereas this abnormal behavior occurs during a fall activity”. This pre-impact fall detection algorithm can be implemented in a wearable fall injury minimization system to track a user's body movement and notify the fall impact reduction device when to activate in order to reduce the severity of the fall injury.